Non-Gaussianities from ekpyrotic collapse with multiple fields

Details Non-Gaussianities from ekpyrotic collapse with multiple fields Non-Gaussianities from ekpyrotic collapse with multiple fields

2007-08-31

arxiv.org/abs/0708.4321v3

JCAP 0711:024,2007

Physics

High Energy Physics

High Energy Physics - Theory

22 pages, 1figure, version revised after journal publication: sign error corrected in the value of non-linear parameter, fNL.

Scientific paper

10.1088/1475-7516/2007/11/024

We compute the non-Gaussianity of the curvature perturbation generated by ekpyrotic collapse with multiple fields. The transition from the multi-field scaling solution to a single-field dominated regime converts initial isocurvature field perturbations to an almost scale-invariant comoving curvature perturbation. In the specific model of two fields, $\phi_1$ and $\phi_2$, with exponential potentials, $-V_i \exp (-c_i \phi_i)$, we calculate the bispectrum of the resulting curvature perturbation. We find that the non-Gaussianity is dominated by non-linear evolution on super-Hubble scales and hence is of the local form. The non-linear parameter of the curvature perturbation is given by $f_{NL} = 5 c_j^2 /12$, where $c_j$ is the exponent of the potential for the field which becomes sub-dominant at late times. Since $c_j^2$ must be large, in order to generate an almost scale invariant spectrum, the non-Gaussianity is inevitably large. By combining the present observational constraints on $f_{\rm NL}$ and the scalar spectral index, the specific model studied in this paper is thus ruled out by current observational data.

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